1. Field of the Invention
The invention relates to field emission devices comprising carbon nanotubes.
2. Discussion of the Related Art
Currently-used vacuum microelectronic devices include flat panel displays, klystrons and traveling wave tubes used in microwave power amplifiers, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. A desirable source of electrons in such devices is field emission of the electrons into vacuum from suitable cathode materials. A typical field emission device comprises a cathode including a plurality of field emitter tips and an anode spaced from the cathode. A voltage applied between the anode and cathode induces the emission of electrons towards the anode.
One promising application for field emitters is thin matrix-addressed flat panel displays. See, for example, Semiconductor International, December 1991, 46; C. A. Spindt et al., xe2x80x9cField Emitter Arrays for Vacuum Microelectronics,xe2x80x9d IEEE Transactions on Electron Devices, Vol. 38, 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, Vol. 83 (1992); and J. A. Costellano, Handbook of Display Technology, Academic Press, 254 (1992). A conventional field emission flat panel display comprises a flat vacuum cell, the vacuum cell having a matrix array of microscopic field emitters formed on a cathode and a phosphor coated anode on a transparent front plate. Between cathode and anode is a conductive element called a grid or gate. The cathodes and gates are typically intersecting strips (usually perpendicular strips) whose intersections define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (e.g., 400 to 5000 eV) to the emitted electrons. See, for example, U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, the disclosures of which are hereby incorporated by reference.
Field emission is also used in microwave vacuum tube devices, such as power amplifiers, which are important components of modern microwave systems, including telecommunications, radar, electronic warfare, and navigation systems. See, e.g., A. W. Scott, Understanding Microwaves, John Wiley and Sons, 1993, Ch. 12. Semiconductor microwave amplifiers are also available, but microwave tube amplifiers are capable of providing microwave energy several orders of magnitude higher than such semiconductor amplifiers. The higher power is due to the fact that electrons are able to travel much faster in a vacuum than in a semiconductor material. The higher speed permits use of larger structures without unacceptable increase in transit time, and the larger structures provide greater power.
A variety of characteristics are known to be advantageous for cathode materials of field emission devices. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from commercially available integrated circuits. For typical device dimensions (e.g. 1 xcexcm gate-to-cathode spacing in a display), a cathode that emits at fields of 25 V/xcexcm or less is generally desirable for typical CMOS driver circuitry. The emitting current density is desirably in the range of 1-10 mA/cm2 for flat panel display applications and  greater than 100 mA/cm2 for microwave power amplifier applications. The emission characteristics are desirably reproducible from one source to another and desirably stable over a very long period of time (e.g., tens of thousands of hours). The emission fluctuations (noise) are desirably small enough to avoid limiting device performance. The cathode is desirably resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing. Finally, the cathode manufacturing is desirably inexpensive, e.g. having no highly critical processes and being adaptable to a wide variety of applications.
Conventional cathode materials for field emission devices are typically made of metal (such as Mo) or semiconductor material (such as Si) with sharp, nanometer-sized tips. While useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of their high work functions. The high voltage operation increases the damaging instabilities caused by ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to be supplied from an external source to produce the required emission current density. In addition, the fabrication of uniform sharp tips is often difficult, tedious and expensive, especially over a large area. The vulnerability of these materials in a real device operating environment to phenomena such as ion bombardment, reaction with chemically active species, and temperature extremes is also a concern.
For microwave tube devices, the conventional source of electrons is a thermionic emission cathode, typically formed from Ir-Re-Os alloys or oxides such as BaO/CaO/SrO or BaO/CaO/Al2O3, which are coated or impregnated with metals, e.g., tungsten. These cathodes are heated to above 1000xc2x0 C. to produce sufficient thermionic electron emissions (on the order of amperes per square centimeter). However, the need to heat these thermionic cathodes has the potential to create problems. Heating tends to reduce cathode life, e.g., by evaporating barium from the cathode surface. Some traveling wave tubes, for example, have lifetimes of less than a year. Heating also introduces warm-up delays, e.g., up to about 4 minutes before emission occurs, and such delays are commercially undesirable. Also, the high temperature operation requires bulky, ancillary equipment, e.g., cooling systems.
Attempts to provide improved emitter materials have recently shown carbon materials to be potentially useful as electron field emitters. Diamond emitters and related emission devices are disclosed, for example, in U.S. Pat. Nos. 5,129,850, 5,138,237, 5,616,368, 5,623,180, 5,637,950 and 5,648,699 and in Okano et al., Appl. Phys. Lett., Vol. 64, 1994, 2742; Kumar et al., Solid State Technol., Vol. 38, 1995, 71; and Geis et al., J. Vac. Sci. Technol., Vol. B14, 1996, 2060. While diamond offers advantages as field emitters due to its negative or low electron affinity on its hydrogen-terminated surfaces, further improvements are desired.
Another, recently discovered carbon material is carbon nanotubes. See, e.g., S. Iijima, xe2x80x9cHelical microtubules of graphitic carbon,xe2x80x9d Nature Vol. 354, 56 (1991); T. Ebbesen and P. Ajayan, xe2x80x9cLarge scale synthesis of carbon nanotubes,xe2x80x9d Nature, Vol. 358, 220 (1992); S. Iijima, xe2x80x9cCarbon nanotubes,xe2x80x9d MRS Bulletin, 43 (November 1994); B. Yakobson and R. Smalley, xe2x80x9cFullerene Nanotubes: C1,000,000 and Beyond,xe2x80x9d American Scientists, Vol. 85, 324 (1997), the disclosures of which are hereby incorporated by reference. Nanotubes take essentially two forms, single-walled (having tubular diameters of about 0.5 to about 10 nm), and multi-walled (having tubular diameters of about 10 to about 100 nm). The use of such nanotubes as electron field emitters is disclosed, for example, in German Patent No. 4,405,768; Rinzler et al., Science, Vol. 269, 1550 (1995); De Heer et al., Science, Vol. 270, 1179 (1995); De Heer et al., Science, Vol. 268, 845 (1995); Saito et al., Jpn. J. Appl. Phys., Vol. 37, L346 (1998); Wang et al., Appl. Phys. Lett., Vol. 70, 3308 (1997); Saito et al., Jpn. J. Appl. Phys., Vol. 36, L1340 (1997); and Wang et al., Appl. Phys. Lett., Vol. 72, 2912 (1998), the disclosures of which are hereby incorporated by reference. Carbon nanotubes feature high aspect ratio ( greater than 1,000) and small tip radii of curvature (xcx9c10 nm). These geometric characteristics, coupled with the relatively high mechanical strength and chemical stability of the tubules, indicate the potential usefulness of carbon nanotubes as electron field emitters. However, carbon nanotubes are generally available only in forms such as loose powders or porous mats, both of which are difficult to incorporate into a device structure. In addition, while previous work has discussed aligning nanotubes in an attempt to improve properties, the alignment has only been performed by techniques which do not appear to be commercially feasible (see, e.g., De Heer et al., Science, Vol. 268, 845 (1995)).
Thus, vacuum microelectronic devices based on improved electron field emitting material are desired. In particular, devices containing carbon nanotube emitters are desired, where the nanotubes are capable of being incorporated into such devices more easily than in current techniques.
The invention provides improved field emission devices containing carbon nanotube electron field emitter structures. According to the invention, adherent carbon nanotube films (containing single-walled and/or multi-walled nanotubes) are disposed on relatively flat conductive substrates. (Adherent film indicates a continuous film having a thickness of 0.1 to 100 xcexcm and having an adhesion strength of at least 1.0 kpsi, as measured by a conventional stud pull test using 0.141 inch diameter studs. Nanotube film refers to a film containing at least 50 volume percent nanotubes.) Previously, attaining even moderate adherence of powdery or mat-like nanotubes to a substrate was difficult, because of the perfect fullerene structure of nanotubes, which tend to exhibit no dangling bonds or defect sites where chemical bonding is able to occur. The invention overcomes these problems, and provides a strongly adherent nanotube film. In addition, it is possible for a portion, e.g., at least 50 vol. %, of the nanotubes in the film to be aligned in substantially the same direction, with their long axes oriented perpendicular to the substrate surface, in order to enhance their emission properties. (Aligned in substantially the same direction indicates that an x-ray rocking curve will exhibit a full-width-at-half-maximum of less than 90xc2x0, for the peak representing inter-shell spacing for multi-walled nanotubes, or for the peak representing inter-tube spacing within a bundle for single-walled nanotubes.)
In one embodiment of the invention, single-walled carbon nanotubes are deposited on substrates that contain a material reactive with carbon, such as carbon dissolving elements (e.g., Ni, Fe, Co) or carbide forming elements (e.g., Si, Mo, Ti, Ta, Cr). When depositing the nanotube film onto such a substrate, it is advantageous to adjust the nanotube formation process such that a high concentration of amorphous carbon (a-C), relative to nanotubes, is initially produced and reacts with the substrate. The process is gradually adjusted to increase the nanotube production, such that the nanotubes are formed with interspersed a-C at the substrate/film interface that anchors the nanotubes to the substrate.
It is also possible to mix pre-formed nanotubes with solvent to form a slurry and then deposit the slurry, e.g., by spin-on, spray, or printing techniques, onto a substrate having a surface layer containing carbon-dissolving or carbide forming materials. It is also possible to use a substrate having a low melting point material, i.e., less than 700xc2x0 C., such as aluminum. Subsequent heating induces either reaction of nanotubes with the carbon-dissolving or carbide forming materials or melting of the surface layer, such that the nanotubes are anchored to the substrate. It is also possible to form an adherent nanotube film by techniques such as mixing pre-formed nanotubes with solvent and binder, and optionally solder, and depositing the mixture onto a substrate. Subsequent heating will activate the binder and/or melt the solder to anchor the nanotubes to the substrate.
In the above embodiments, it is possible to simultaneously align the nanotubes in substantially the same direction by depositing them in a magnetic or electric field, such that the anisotropic nanotubes align their long axes with the field lines during deposition. It is believed that alignment of the nanotubes provides improved emission properties due to more efficient and effective field concentration at the aligned tubule ends. Alignment of pre-formed nanotubes is also capable of being achieved by mixing nanotubes with a conductive polymer to form a composite material, and then straining the composite with a uniaxial load. (A conductive polymer exhibits an electrical resistivity less than 1 ohm-cm.) It is then possible to adhere the composite to a substrate.
The invention thereby provides a device containing an improved carbon nanotube film emitter structure, due to the nanotube film""s strong adherence to a substrate and optional alignment in a substantially uniform manner. Such nanotube emitters show desirable emission properties, e.g., low threshold voltage (about 3-4 V/xcexcm or less at a current density of 10 mA/cm2), high current densities (greater than 0.2 A/cm2) and excellent reproducibility and durability. In addition, the emission characteristics appear to remain essentially the same even after the emitting surface is exposed to air for several months.